The present invention is related in general to wireless communications systems, and more particularly to an improved method and system for estimating adaptive array weights used to transmit a signal to a receiver in a wireless communication system.
A frequent goal in designing wireless communications systems is to increase the number of users that may be simultaneously served by the communications system. This may be referred to as increasing system capacity.
In code division multiple access (CDMA) wireless communications systems, the use of adaptive antenna arrays at the base transceiver has been proposed as a method of increasing system capacity. An adaptive array antenna includes two or more radiating elements having dimensions, spacing, orientation, and illumination sequences such that the fields for the individual elements combine to produce a field having greater intensities in some directions and lesser field intensities in other directions. An adaptive array antenna helps increase system capacity because this field pattern, or radiation pattern (which may include a plurality of beams or lobes), may be configured such that signals intended for a selected user are in higher-gain antenna lobes that are pointed in the direction of a propagation path to a selected user, while nulls in the antenna pattern are likely to be directed to other, non-selected users. Thus, a signal intended for the selected user may be directed toward a non-selected user with a lower power level. This increases capacity because the selected user""s signal is not transmitted with a higher antenna gain to all the non-selected users in the sector or cell, thus not degrading the receiver performance of the non-selected users. While some non-selected users may be in a higher gain lobe, others are not, which makes all users statistically better able to receive their intended signals.
In prior art proposals for adaptive array transmitters that adjust their patterns on a per user basis, a high-power, per-user pilot is typically used. This is because proper coherent demodulation requires a pilot that is in phase with the traffic channel. Thus, if the pilot is not transmitted with the same antenna pattern as the traffic channel, the pilot phase will be shifted relative to the phase of the traffic channel, which means that it cannot be used as a reference for demodulation. In an adaptive array system having a pilot for each user, each user""s pilot is added to the user""s traffic channel, and modified in accordance with the weights (i.e., the gains and phases) used for creating the user""s traffic channel illumination sequence.
Disadvantages of adding a pilot to each user""s signal, where the pilot power is high enough to use a as a demodulation reference, include: (1) diminishing system capacity due to system overhead caused by each user having a pilot; (2) requiring longer pilot sequences to distinguish each pilot; (3) increasing the complexity of the pilot searcher in the subscriber unit due to the longer pilot sequences; (4) eliminating backward compatibility with existing CDMA cellular (IS-95) subscriber units; and (5) increasing soft handoff complexity because of the assignment of an additional pilot per user during soft handoff.
Adding a high-power per-user pilot for a demodulation reference may effectively eliminate the gain obtained by using an adaptive array. If we assume that a broadcast pilot takes up 7% of the total transmit power, and that per-user pilots use the same 7% of total transmitted power, 7% capacity is lost because broadcast pilots are still required for handoff purposes.
In order to demodulate a signal transmitted from an adaptive antenna array without a high-power per-user pilot, a pilot must be created, or synthesized in the receiver to provide a phase reference for the demodulator. To synthesize a pilot, it is helpful to know what xe2x80x9cweightsxe2x80x9d were used at the transmitter to generate the signals applied to each antenna in the antenna array. These weights represent the set of gain and phase modifications made on a user""s signal to generate each antenna element signal. The antenna element signals work together to form the transmission pattern of the antenna array.
One method of informing the receiver of the weights used at the transmitter is to send a message representing the weights. In a system that rapidly changes weights, the overhead of sending so many messages reduces system capacity for transmitting user traffic. Additionally, such a message stream will have an error rate, and an error in the weight message may further deteriorate capacity by causing the receiver to ask for more power because the wrong weight assumptions were used to synthesize a pilot, causing errors in demodulating a signal that otherwise had enough power for correct demodulation.
In the absence of weight messages, the ability to estimate adaptive array transmission weights used at the transmitter is useful in the pilot synthesizing process in the receiver. Weight estimation avoids the overhead of messaging and may avoid the consequences of erroneously receiving a weight message. Thus, it should be apparent that a need exists for a method and system of estimating adaptive array weights used to transmit a signal to a receiver in a wireless communication system, wherein the transmitter does not use high-power, per-user pilots or high-capacity, complex signaling between the base transceiver and the subscriber unit.